It is 30 days from the first minute the original DROS is run or 720 hours. It only applies to handguns. Longuns can be purchased every day without time restrictions.
Example: You buy a handgun at 10:00 AM on August 1st (time and date the DROS was run) and on August 31st at 10AM you can purchase another and DROS.
There is nothing stoping you from buying 5 handguns at a time but you can only DROS and pickup a handgun one every 30 days or 720 hours